1. Field of the Invention
The present invention relates to a high pressure discharge lamp starter device for turning on an essentially mercury free high pressure discharge lamp and an automotive headlight device using the high pressure discharge lamp starter device.
2. Description of the Related Art
Japanese Patent Application KOKAI Publication No. 11-238488 (Prior Art 1) discloses an essentially mercury free metal halide discharge lamp charged with a discharge medium containing a first halide, which is a halide of a desired light-emitting metal, a second halide, which is a halide of a metal rarely emitting light in a visible range compared to the metal of the first halide, and a rare gas. The metal halide discharge lamp, even though it is virtually free from mercury of large environmental-burdening unlike a conventional metal halide discharge lamp, is excellent since the lamp voltage of the metal halide discharge lamp can be increased to supply a desired lamp power. In this respect, the high pressure discharge lamp is advantageous. Note that Japanese Patent Application KOKAI Publication No. 11-86795 discloses substantially the same technique as Prior Art 1 is disclosed.
Furthermore, a mercury-free automotive metal halide lamp containing an NaI—ScI3 based metal halide plus InI is disclosed in the research material for the 15th conference on light-source physical characteristic application, page 69 to 73 (Prior Art 2) held on Dec. 4, 2000 (published by the Illuminating Engineering Institute).
On the other hand, in high pressure discharge lamps such as metal halide lamps used in automotive headlights, which requires not lower than a predetermined amount of light in a short time, a technique is known for reducing the light-amount rise time by supplying a lamp power larger than the rated lamp power during low-temperature startup time. Typically, in the beginning of the startup time, a lamp power of about 70 W is supplied to a lamp whose rated lamp power is 35 W. The lamp power is reduced with time or with an increase of the lamp voltage. Finally, the lamp power is settled at the rated lamp power. In this case of a conventional mercury-containing high pressure discharge lamp (hereinafter, referred to as a “mercury-containing lamp” for convenience sake), mercury having a low boiling point is first vaporized to emit light and then a halide of a light-emitting metal is vaporized as the lumen temperature increases, contributing to the initial light emission. For this reason, if light emitted from mercury is equal to or more than a predetermined amount in the beginning to the startup, a relatively long time can be given until the initial light-emission from a light emitting metal takes place. To explain more specifically, immediately after the startup, even if a large power of about 70 W is supplied to a lamp for light emission, the mercury vapor pressure is saturated in about two seconds. Thereafter, even if a lamp power less than 70 W is supplied, the same light amount as in the stable light-emission time can be expected. In short, it can be take several tens seconds from the initiation of vaporization of a halide of a light-emitting metal until the light emission from the light-emitting metal becomes predominant. Because of this, high pressure discharge lamps, even if they vary in shape, content of a light-emitting metal halide, and distribution of the halide attached to the lumen, can obtain relatively stable light-amount rise characteristics.
In contrast, in a high pressure discharge lamp containing no mercury (hereinafter, referred to as a “mercury-free lamp” for convenience sake), since light emission from mercury cannot be expected in the beginning of the startup, first a rare gas such as xenon emits light and then a light-emitting metal contained in the form of a halide initiates light emission.
However, in the mercury-free lamp, an extremely high temperature is required to vaporize a halide of the light-emitting metal compared to mercury. Because of this, to obtain the same light amount as in a mercury-containing lamp in the same time, a lamp power not less than 2.5 times as large as the rating lamp power must be supplied for further longer time than in the mercury-containing lamp. However, when such high lamp power is supplied, a metal halide contained in the lamp abruptly vaporizes, with the result that the light emission amount increases twice to thrice that of a stable light-emission under the supply of rated lamp power.
FIG. 1 is a graph showing change of lamp voltage (VI), lamp current (II) and output of light (L) during the startup time when a lamp power 2.5 times as large as rated lamp power is supplied to a mercury-free lamp. In the graph, the horizontal axis indicates time and the vertical axis indicates each of the magnitudes of V1, II and L. The light emission L sharply increases in a certain region, which means that a metal halide abruptly vaporizes.
In a mercury-free lamp, when intensive vaporization of a metal halide takes place in the absence of any control as described above, a large amount of light significantly in excess of rated light is emitted. As a result, in the case of an automotive headlight device, the driver of an oncoming car will be strongly dazzled. This is extremely dangerous.
Furthermore, in a mercury-free lamp, since no light is emitted from mercury, even if the same lamp power is supplied, the variation of light-amount rise characteristics of high pressure discharge lamps becomes more apparent. Such variation in light-amount rise characteristics is ascribed to difference in shape of a high pressure discharge lamp, the content of a light-emitting metal halide, and the distribution state of the light-emitting metal halide attached to the lumen. Because of this, time from the startup until a metal halide abruptly vaporizes varies.
To deal with the aforementioned problems, it is considered that the supplied lamp power is manipulated to reduce at the same time the intensive vaporization of a metal halide takes place by predicting the initiation time of the intensive vaporization. However, if the lamp power supply is reduced before a metal halide starts intensive vaporization under the influence of variation in light amount rise characteristics, the light emission amount considerably decreased to a level lower than that of the stable light-emission time on the contrary to the above case. As a result, the illuminance given to a road surface becomes insufficient. This is also extremely dangerous.